This invention relates to the acquisition and processing of data acquired by a measurement-while-drilling (MWD) tool during the drilling of a wellbore. More particularly, the invention relates to methods and devices for acquiring data downhole using a tool that is adapted to be clamped to the borehole wall during drilling operations.
Modem well drilling techniques, particularly those concerned with the drilling of oil and gas wells, involve the use of several different measurement and telemetry systems to provide petrophysical data and data regarding drilling mechanics during the drilling process. Data is acquired by sensors located in the drillstring near the bit and either stored in downhole memory or transmitted to the surface using MWD telemetry devices. Prior art discloses the use of downhole devices incorporating resistivity, gravity, magnetic and nuclear magnetic resonance measurements on a rotating drillstring.
Prior art devices are limited to measurement devices that rotate with the drillstring. This is particularly problematic in nuclear magnetic resonance (NMR) measurements where lateral vibrations of a drill collar containing the NMR device would adversely affect an NMR measurement. For example, a lateral, 50 Hz vibration of 1-mm amplitude (100-g acceleration) would disable a typical device with a resonance region of the order of 1 mm. Furthermore, since the drillstring can make anywhere between 0.1 to several rotations in the duration of a pulsed NMR measurement (on the order of 0.01 to 1 second), an NMR device on a drillstring must be rotationally symmetric. Prior art NMR devices in which the static magnetic fields are produced by magnets located in the drilling collar suffer from the additional disadvantage that the resonance region extends into the borehole, as a result of which an electromagnetic signal is produced in the borehole fluid. The resulting electromagnetic signal of the borehole fluid must be canceled because the pulsed NMR device functions by detecting protons in fluids. Typically, a porous rock formation may contain 10% fluid by volume whereas the borehole fluid contains more than 50% fluid and has a high density of protons. As a result of this, the electromagnetic signal of the borehole fluid would dominate any formation signal detected by the pulsed NMR device and a special arrangement is necessary to cancel the borehole fluid signal. The present invention overcomes these inadequacies.
The present invention is an apparatus and method of determining a parameter of interest of a formation surrounding a borehole while drilling the borehole. In one aspect of the invention, the drill bit is mounted on a rotating drillstring or coiled tubing. The present invention includes a longitudinal member, for example, a segment of drill pipe included in the drillstring and rotating with the drillstring, or a shaft on a downhole directional drilling assembly. A sensor assembly is slidably coupled to the longitudinal member wherein the sensor assembly includes at least one sensor for obtaining measurements relating to the parameter of interest. When the sensor assembly is held in a non-rotating position, for instance, for obtaining the measurements, the longitudinal member is free to rotate and continue drilling the borehole. The sensor assembly is slidably coupled to the longitudinal member using, for example, at least one guide sleeve slidably coupled to the longitudinal member. The sensor assembly further includes, for example, at least one transmitter. The sensor assembly of the present invention can include any of a variety of sensors and/or transmitters for determining a plurality of parameters of interest including, for example, nuclear magnetic resonance measurements.
Returning drilling fluid flows outside the sensor assembly, or alternatively, a flow path between the sensor assembly and the longitudinal member allows for the flow of the drilling fluid. In a number of embodiments, at least one clamping device engages the borehole, when activated, for engaging the borehole walls and holding the sensor assembly in the non-rotating position. When the clamping device is deactivated, the sensor assembly disengages from the borehole and the sensor assembly moves to another location in the borehole wherein the clamping device is activated. The sensor or at least one transmitter can be located in the clamping device to make contact with the borehole wall and lock the sensors in place when the sensor assembly is clamped. The clamping device is hydraulically, mechanically, or electrically activated.
The sensor assembly is held against gravitational pull and provided for axial movement using a support device such as a spring device fixedly attached to the longitudinal member, or a hydraulic cylinder fixedly attached to the longitudinal member. In another embodiment, the present invention includes a belt drive device for holding the sensor assembly in the non-rotational position, and for providing a non-continuous movement of the sensor assembly relative to propagation of the longitudinal member.
In still another embodiment, the sensor assembly further includes a sensor for providing azimuthal measurements and determining a tool face orientation of the sensor assembly, and further including a rotational positioning control device for positioning the sensor assembly to a desired tool face orientation.
In still another embodiment, at least one thruster is connected to the sensor assembly for providing axial decoupling and dampening vibrations to the sensor assembly. At least one knuckle joint can also be connected to the thruster to provide further axial decoupling and dampening.
In another embodiment, the sensor assembly is slidably coupled to the longitudinal member using at least two stabilizers on the drillstring connected to the sensor assembly through at least one shaft. The sensor assembly also includes a clamping device to hold the sensor in the non-rotating position.
In each embodiment, magnetic and inertial sensors can be used to provide information on the orientation of the measurement sensors. A telemetry system, for example, sends information downhole about the depth of the drilling assembly. A microprocessor downhole combines the depth and azimuth information with the measurements made by the rotating sensors, uses redundancy in the data to improve S/N ratio, compresses the data and sends it uphole by a telemetry system or stored downhole for later retrieval.
In another aspect of the invention, the drill bit is driven by a downhole drilling motor. The motor may be on a rotating drillstring or on coil tubing. In any of these arrangements, the parameters of interest include NMR characteristics of the formation, resistivity, density, compressional and shear wave velocity and structure, dipmeter and acoustic porosity.